1. Field of the Invention
The present invention relates to a substrate for forming an electron source, an electron source using the substrate, and an image display device using the electron source.
2. Description of the Related Art
Two types of electron emitting devices, i.e., thermionic-cathode devices and cold-cathode devices, have been known. For example, surface-conduction-type devices, field-emission-type devices, metal/insulating layer/metal-type devices have been known as the cold-cathode devices.
The surface-conduction-type devices utilize the phenomenon that electron emission occurs by causing a current to flow in a direction parallel to the surface of a small-area thin film formed on a substrate. In the surface-conduction-type devices, electron emitting portions are formed by performing current-supply processing, called current-supply forming, on a conductive film before performing electron emission. That is, the current-supply forming processes supply current by applying a constant DC voltage or a DC voltage that increases at a very slow rate between both ends of a conductive film, to locally destruct or alter the conductive film in order to form electron emitting portions that have a high electric resistance. Cracks are generated at locally destructed, deformed or altered portions of the conductive film. When an appropriate voltage is applied to the conductive film after the current-supply forming, electron emission occurs at portions near the cracks.
An electron-source device includes the above-described electron emitting devices formed on a substrate wherein the electron emitting devices are wired in the form of a simple matrix by a plurality of row-direction wire electrodes and a plurality of column-direction wire electrodes. Particularly, an insulating layer is formed between electrodes at each of portions where the row-direction wire electrodes and the column-direction wire electrodes cross, in order to maintain electrical insulation. The above-described conductive film is formed in order to form electron emitting portions. By supplying current by applying a constant DC voltage or a DC voltage increasing at a very slow rate between both ends of the conductive film, the conductive film is locally destructed or altered in order to form electron emitting portions having a high electric resistance.
A phosphor film including phosphors is formed on a surface of a faceplate (a light-emitting display plate), connected so as to face the electron-source device, opposite to the electron-source device, and phosphors of three primary colors, i.e., red, green and blue, are appropriately coated. A black substance is provided between phosphors constituting the phosphor film, and a metal back made of Al or the like is formed on the phosphor film. The inside of an envelope obtained by connecting the faceplate and the electron-source device using a supporting frame is maintained to a vacuum of about 10−6 Torr. In order to provide the substrate with a strength sufficient enough to resist against the atmospheric pressure, a structural supporting member comprising a relatively thin glass plate is provided.
In an image display device in which phosphors are caused to emit light by projecting an electron beam emitted from an electron source onto an appropriate one of phosphors, serving as image display members, it is necessary to maintain the inside of an envelope including the electron source and the image display members of the faceplate in a high vacuum. This is because if a gas is generated within the envelope to increase the pressure within the envelope, the gas adversely influences the electron source to reduce the amount of electron emission although the degree of the influence depends on the type of the gas, and it becomes impossible to display a bright image. In some cases, the generated gas is ionized by the electron beam and damages the electron source due to the collision of the ionized gas with the electron source by being accelerated by an electron field for accelerating electrons. Alternatively, discharge may occur within the envelope, to sometimes destruct the device.
Usually, the envelope of the image display device is assembled by using a glass supporting frame and bonding connection portions by frit glass or the like, and the inside of the envelope is evacuated to a vacuum of about 10−7 Torr by connecting the inside of the envelope to a vacuum pump via an exhaust tube. Then, the exhaust tube is sealed. The vacuum after the sealing is maintained using a getter provided within the envelope. That is, a getter film is formed at a predetermined position within the envelope. The getter film is formed by heating and evaporating a getter material having, for example, Ba as a main component according to high-frequency heating. The inside of the envelope is maintained at a vacuum of about 10−6 Torr according to the adsorption function of the getter film.
In the above-described image display device, if a voltage is applied to electron emitting devices via external terminals, provided outside of the envelope, via the plurality of row-direction wire electrodes and the plurality of column-direction wire electrodes formed on the substrate for forming an electron source, an electron beam is emitted from each of the electron emitting devices. At the same time, by applying a high voltage of several tens of kV to the metal back formed on the phosphor film of the faceplate via a terminal provided outside of the envelope, the emitted electron beam is accelerated to impinge onto the phosphor film of the faceplate. The phosphors of the respective colors constituting the phosphor film are thereby excited to emit light, so that an image is displayed.
In some cases, in order to block diffusion of Na, a layer having SiO2 as a main component is formed on a substrate containing Na, and an antistatic layer is formed on the substrate in order to prevent charging on the surface of the substrate.
However, the above-described image display device has the following problems.
First, the above-described coated layer formed on the substrate for forming an electron source may cause difficulty in maintaining the high-vacuum state within the envelope formed by connecting the substrate and the faceplate via the supporting frame, depending on the state of formation of the coated layer. It is estimated that this is because the inside of the coated layer may have gas permeability.
Second, the getter provided on the coated layer within the envelope in order to maintain a vacuum within the envelope may cause a short circuit between adjacent wire electrodes, even if the coated layer is made of an insulator. It is estimated that this is because a large number of bubbles are sometimes formed in the coated layer depending on the state of formation of the coated layer, and the bubbles are burst during heating at a high temperature to provide a state in which the wire electrodes are exposed. This short circuit may greatly degrade the quality of the formed image. Hence, in the worst case, the production yield of the image display device is degraded by manufacturing failed products.